The present invention relates to oxygen ion conductors and, more particularly, to such a device for sensing oxygen partial pressure in a first gas relative to a known oxygen partial pressure in a second gas, as well as for use as a fuel cell.
Because oxygen is an essential element of combustion processes, it is helpful to use oxygen sensors to monitor the exhaust gases of the combustion. Optimum fuel efficiency of a flame requires the correct oxygen-to-fuel ratio, since too little oxygen results in wasted, unburned fuel. Moreover, too great an oxygen concentration also wastes fuel, since energy is used to heat the excess oxygen to the exit gas temperature. This problem is compounded, since air is generally used as a source of oxygen in combustion processes, consisting approximately of 20% oxygen and 80% nitrogen by volume. For each unit volume of excess oxygen heated to the flame temperature, four units of nitrogen must be heated as well.
The energy savings potential available through the use of oxygen sensors is well known. Presently known sensors function by monitoring the EMF developed across an oxygen ion conductor which is exposed to different partial pressures of oxygen.
Oxygen tends to move from a gas containing a high concentration of oxygen to one of lower concentration. If the two gases are separated from each other by an oxygen ion conductor, the oxygen molecules will dissociate on one surface of the conductor and absorb electrons to form oxygen ions. These ions can then diffuse through the ionic conductor, leaving the entry surface with a deficiency of electrons. On the exit or low oxygen concentration side of the conductor, oxygen ions leaving the material must give up electrons to form molecular oxygen, leaving the exit surface with an excess of electrons. Thus, an electrical potential difference, or EMF, is set up between the two surfaces of the ion conductor. The greater the difference in oxygen content of the two gases, the greater will be the tendency of oxygen to diffuse through the conductor, and the greater will be the potential difference between the entry and exit surfaces.
The EMF generated by the difference in partial pressures may be calculated by the Nernst relation: EQU EMF=t.sub.i (RT/nF) ln (P.sub.O.sbsb.2 /P'.sub.O.sbsb.2) (1)
where t.sub.i is the ionic transference number, R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the electrode reaction, F is the Faraday constant, and P.sub.O.sbsb.2 and P'.sub.O.sbsb.2 are the oxygen partial pressures in the first and second gases, respectively. In the present case, the electrode reaction is O.sub.2 +4.sub.e .fwdarw.20.sup.-2, and thus n=4.
A common known oxygen sensor is disclosed in A. M. Chirino and R. T. Sproule, "Application of High and Low Temperature Direct Continuous Oxygen Sensors", 59 Am. Ceramic Soc. Bull. 605 (1980). The oxygen ion conductor material used therein is a solid solution of ZrO.sub.2 and approximately 8% by weight Y.sub.2 O.sub.3. The addition of the Y.sub.2 O.sub.3 to the ZrO.sub.2 provides vacant oxygen sites in the material which are necessary for diffusion of the oxygen ions. The stabilized ZrO.sub.2 is formed into the shape of a tube closed at one end, the tube typically being approximately 3/8 inch (1 cm) in diameter with wall thickness of 0.050 inches (0.125 cm) and 6 inches (15 cm) in length. The outer surface of the tube is exposed to the combustion exhaust gases, generally having a low oxygen content. The inner surface is simultaneously exposed to a reference atmosphere, usually atmospheric air, containing a higher oxygen content. Both surfaces of the tube are covered with a porous platinum metal coating which allows the gaseous oxygen to reach the ZrO.sub.2 and provides an electrical contact for measurement of the generated EMF.
Calcia-stabilized ZrO.sub.2 tubes have also been constructed, although the yttria-stabilized tubes are preferred due to their higher ionic conductivity and lower activation energy. Similarly, the use of ceria (CeO.sub.2) ceramic tubes has also been studied.
There are several disadvantages to the ZrO.sub.2 and CeO.sub.2 electrolyte tubes. Raw material costs are relatively high, particularly in the case of yttria-stabilized ZrO.sub.2. Moreover, very high temperatures are required for the ceramic processing of ZrO.sub.2 and CeO.sub.2, which have melting points of 2700.degree. C. and 2600.degree. C., respectively. The combination of these two disadvantages results in relatively high tube costs.
The long term usefulness of the zirconia-based electrolytes may be lost due to destabilization or the growth of monoclinic grains into the fluorite grains, either of which leads to degraded signal output. Additionally, the geometry of the tube leads to large internal resistances in the sensor requiring that the electrolytes be heated to approximately 600.degree. C. to reduce the internal resistance below the external load resistance. Moreover, the entire, platinum-coated outer surface of the electrolyte tube is exposed to the exhaust gas which can result in undesirable corrosion and/or erosion problems; and the relatively small thermal conductivity of the stabilized zirconia can lead to thermal shock problems on thermal cycling, particularly with large tubes.
What is needed, therefore, is an oxygen sensor that is relatively inexpensive to manufacture, has a relatively long useful life, and requires a lower threshold operating temperature. The sensor should also be less susceptible to corrosion and/or erosion attack on the precious-metal electrodes, and should possess a high resistance to thermal shock.